1. Technical Field
This invention relates to disposable, thin-wall, plastic lids, and the mold for making such lids. More particularly, this invention relates to a novel lid for use in closing containers of the type having a peripheral bead about the open end thereof.
2. Background Art
Due to the rapidly growing use of disposable, thin-wall, plastic lids in fast food establishments, vending machines and automatic filling equipment, the need has arisen for a lid whose design permits a plurality of such lids to be formed into a stack having unsupported lateral stability while preventing individual lids from jamming when the stack is subjected to axially applied loads. Early attempts to solve this need include the provision of stacking facilities in the form of radial undercuts in the side wall of the lid whereby the undercuts form internal and external shoulders that cooperate with identical stacking facilities in adjacent lids so that the internal shoulder of the upper lid would rest on the external shoulder of a lower lid of the stack. U.S. Pat. No. 3,379,340 to Silvio discloses a lid of this type. This arrangement prevents lateral displacement of one lid relative to another when the lids are stacked in telescopic relationship. However, this arrangement makes separating the lids and removal of a single lid from the top of the stack difficult at times because the lids can become stuck together. Also these undercuts are generally provided in the skirt of the lid which is particularly thin, lacks rigidity and will deform under excessive loads. Consequently, these undercuts do not always perform as intended.
In solving the problems associated with the above mentioned lid stacks, U.S. Pat. No. 4,275,815 to Davis issued June 30, 1981, has provided a plurality of circumferentially spaced, downwardly extending feet, with each adjacent lid having a different number of feet to keep the lids from becoming aligned and telescoping. These feet are positioned such that the feet of one lid in a stack will rest on a depressed central portion of a lower lid. The feet provide for both resistance against jamming of the successive lids as well as resistance against lateral displacement of adjacent lids. However, due to the circumferential spacing of these feet the lids are capable of lateral movement which is greater than the designed tolerance between the feet and the recess wall. Also, lateral forces exerted on the lids are assumed directly by the feet themselves which can result in deformation of the feet and the subsequent possible unwanted telescoping and jamming of the lids within the stack. Others have suggested anti-jamming lugs for lids wherein the spacing radially or circumferentially of the lugs from one lid to the next is varied, for example U.S. Pat. Nos. 1,786,733 to Benoit; 1,931,595 to Weguelin; and 3,632,016 to Bozek.
Further problems arise when lids of the dome-type are utilized as opposed to those having a central depression as is incorporated in the lid disclosed in U.S. Pat. No. 4,275,815. As shown in U.S. Pat. No. 3,498,495, to Davis, issued Mar. 3, 1970, ribs can be provided on the external surface of a raised central portion or dome to maintain the outer surface of the dome of one lid of a stack in circumferential spaced relationship with the inner dome surface of the adjacent lid. This spacing does allow for easier separation of the lids but any axial forces generated during the maneuvering of the stack must be absorbed by the peripheral skirt and the side wall of the ribs which may deform these thin walled members, and cause excessive telescoping of the lids leading to a possible sticking problem.
Lids of the above mentioned type are often provided with a plurality of flutes and lands about their periphery. These flutes may take the shape of diamonds, as shown in U.S. Pat. No. 4,275,815 discussed above, ovals, or these flutes may be rectangular in configuration as shown in U.S. Pat. No. 3,883,036 to Mahaffy et al issued May 13, 1975. Flutes of this type are designed to provide support for the peripheral skirt against crushing in the axial direction and also to provide breaks in the waist which are formed between the lands to allow the peripheral skirt to expand outwardly over the bead of a cup to be covered and snap back below such bead. However, lids having flutes of this type often require excessive force to remove the lids from a mold during the molding process and also provide only a single circumferential zone of support which. depending upon the use of such lids, could possibly be inadequate.